1. Technical Field
The present invention relates to a printer assembly (printing assembly) and a printer for printing on a thermosensible (thermal) paper.
2. Description of the Related Art
In recent years, various types of computers have been developed and used that includes a portable type such as a notebook type and a portable or mobile type such as a PDA which can be put in a pocket. As the Internet becomes popular, applications of computers have been spread in various fields including information service and communication, and the population of users has also been spread from a specialist to a general user. Accordingly, computers will be daily used in ordinary homes in the future. In addition, the application handling daily works with a computer at home, by introducing computerized account settlement, electronic commerce.
According to increase of mobile computers and other mobile devices, user will require printout whenever they are required even if they are using mobile computer. However, in a case that computers are used for daily works such as electronic transaction, an amount of prints is not so many and the frequency of printing is also not so many. Accordingly, one style or aspect of a printer which is demanded in the future for the above mobile computer will surely be thin, compact, light-weight and low-cost type and it can be carried together with a portable information processing device and/or portable terminal such as a PDA, a portable telephone or the like, by being built in the device and/or being connected to the device.
A printer for printing on a thermosensible paper or thermal paper using a thermal head does not require ink or ribbons. Therefore, its printing mechanism becomes compact. Specifically, a line thermal printer having a line thermal head extending in a paper width direction (scanning direction or line direction) does not require a mechanism for moving a thermal head in the scanning direction. Hence the line thermal printer becomes much compact and the printer meets the above demand.
However, a mechanism for pressing a printing paper on a thermal head is an obstacle to realize a very thin printer having a thickness of, for example, about 10 mm or less. In the thermal printer, a mechanism for feeding a thermal paper or sheet has a platen roller. The platen roller presses the paper on the thermal head by holding the paper between the thermal head and feeds the thermal paper by rotating. In order to realize a thin printer having a thickness of 10 mm or less, a diameter of the platen roller shall be smaller than that of the conventional printer. When the diameter of the platen roller becomes small, a contacting area or surface between the roller and the printing paper is reduced, so that the platen roller must be pressed on the paper with stronger force in order to keep a sufficient contacting force for pressing the thermal paper to the thermal head. However, when the diameter of the platen roller becomes 10 mm or less, the roller lacks in strength. Therefore, when pressing force is increased for compensating for the lack of contacting area, flexure or bending tends to occur in the platen roller.
Particularly, when the diameter becomes 5 mm or less, flexure occurs over the entire length of the platen roller. Accordingly, when the platen roller is supported only at both ends, it becomes impossible to press the printing paper on the thermal head by the platen roller with sufficient force for printing. Particularly when the line thermal head is used and the force for pressing the thermal paper is deviated along the direction of the paper width, the pressing forces are lost in some portion of the paper. Therefore, printing quality deteriorates so that it becomes impossible to perform practical printing.
For preventing the platen roller from flexing, the platen roller may be divided along its longitudinal direction so that the shaft of roller is exposed and is supported by a bearing. However, for printing along the thermal head without missing of data, the pressure must be continuously applied by the platen roller along the scanning direction. Therefore, the platen roller cannot be divided along its longitudinal direction. It is also possible to support the entire platen roller from a side opposed to the thermal head by a member extending in the longitudinal direction. However, for feeding the paper smoothly, the surface of the platen roller is formed of material having a high coefficient of friction, such as a rubber. For this reason, in a pressed or pressurized state, a frictional force between such a supporting member and the platen roller is excessively large so that driving force of a motor for rotating the platen roller becomes too large and such a large capacity motor cannot be installed in thin and small printer.
It is also possible to install several sub rollers along the longitudinal direction of the platen roller to prevent the platen roller from flexing. However, if we applied the sub rollers supported by a shaft and bearings, it is necessary to make an additional space for arranging the sub rollers and their bearings including some clearances between the sub rollers and the housing of printer. Therefore, the above solution cannot be applied for the extremely thin printer of this invention.
Hence, an object of this invention is to solve the above problems about the platen rollers that are the neck of realizing a very thin printer and to provide a thin printer that can achieve a high printing quality. In addition, in the present invention, it is an object to provide a very thin printing mechanism or printer assembly and a very thin line thermal printer by the printer assembly that has a thickness about 10 mm or less, and further about 5 mm or less.
For achieving the object, in a printer assembly of the present invention, a platen roller is supported by thin rollers (shafts) of a small diameter having a surface of small or low coefficient of friction. In addition, a plate-shaped member supports or bears the supporting rollers so that flexure of the platen roller is prevented via the supporting plate and supporting rollers. Therefore, in this invention, if the diameter of the platen roller is small, stronger pressure or force can be applied between the platen roller and a thermal head. That is, the printer assembly of the invention comprises a line thermal head, a platen roller having a surface of high or large coefficient of friction, which holds a printing paper between the line thermal head and the platen roller and feeds the printing paper, supporting rollers having a surface of small or low coefficient of friction respectively, which are disposed at a front and a rear of the platen roller in a paper feeding direction so as to support the platen roller from a side opposed to the line thermal head, a supporting plate extending along the supporting rollers for supporting or bearing the supporting rollers from a side opposed to the line thermal head, and pressurizing or pressing means for applying pressure between the supporting rollers and the line thermal head.
The platen roller made of a metal member whose surface is processed for increasing a coefficient of friction or the platen roller made of a resin material with a high coefficient of friction such as phenol resin may be used, but the platen roller covered by the rubber materials having some elasticity and a high coefficient of friction is the most suitable. On the other hand, a member made of resin having a low coefficient of friction, for example, nylon, polyethylene or the like is suitable for the supporting rollers. A rod-shaped member made of fluororesins is the most suitable for the supporting rollers since it has so smaller coefficient of friction that becomes several tenths, or one or more digits smaller than that of rubber material.
In this printer assembly, the supporting rollers having a small coefficient of friction support the platen roller having a large coefficient of friction, and the supporting plate extending along the longitudinal directions of the supporting rollers bears the supporting rollers. Therefore, the supporting plate receives or bears the load of the platen roller continuously along the longitudinal direction of the platen roller via the supporting rollers. Accordingly, even when the platen roller itself is not high in rigidity, it is prevented from flexing and/or bending and applies high pressure on the paper along the width direction (the longitudinal direction of the roller) continuously.
In addition, the platen roller having a surface of high coefficient of friction rotates the supporting rollers having a surface of small coefficient of friction, and the supporting rollers touch and rotate on the supporting plate. Accordingly, frictional force against to the rotation of the platen roller becomes so small.
Furthermore, as the supporting rollers are directly supported or received by the supporting plate, it becomes unnecessary to separate the supporting rollers from a structure member of printer assembly for making a clearance. Therefore, the line thermal head, the platen roller and the supporting rollers can be disposed in a very thin space. Accordingly, in the printer assembly of this invention, the above components are assembled in a very thin space and the driving power for paper feeding becomes so small.
Also, in this printer assembly, the supporting rollers are provided at front and rear of the platen roller. Therefore, the supporting rollers fix the front and rear positions of the platen roller. Furthermore, by this arrangement of the platen roller and the supporting roller, the supporting rollers are not disposed along a line of the thermal head and the platen roller. Namely, a line connecting the center axis of the platen roller and the center axis of each supporting roller is inclined or not parallel to the line of the platen roller and the thermal head. Accordingly, the line thermal head, the platen roller and the supporting rollers are disposed triangular like arrangement in a thinner manner by absorbing or reducing space corresponding to the diameter of the supporting rollers.
For example, for printing a printing paper with a post card size (A6 size 105 mmxc3x97148 mm) or so, a printer assembly having a thickness of about 5 mm or less can be realized by combining a thermal head having the thickness of 2 mm, the platen roller having a diameter of 2 mm and supporting rollers having a diameter of 1 mm. Accordingly, by combining the printer assembly of this invention and a paper feeding mechanism for feeding thermal paper thereto, a compact printer that is very thin and portable can be provided. Therefore, the printer of this invention can be built into information processing device, and being suitable for use together with a portable terminal such as a portable telephone and a PDA, in a docking manner and/or built-in manner.
In the printer assembly, it is preferable that each of the supporting rollers rotates at predetermined position at front and rear of the platen roller. The supporting plate having a U shape inner portion can accommodate the supporting rollers respectively in front and rear corners of the U shape inner portion. Also, the supporting plate having grooves and/or recesses for fixing rotating positions of the supporting rollers can be applicable.
In the printer assembly of this invention, when a printing paper is a larger size, more torsional strength are required for the platen roller and therefore the platen roller having a diameter of about 10 mm is required. However, this printer assembly dose not require a platen roller having rigidity to the extent being not flexed as a conventional printer. Accordingly, a printer assembly and a printer having a thickness of ten-odd mm for printing an A4 size (210 mmxc3x97297 mm) sheet can be realized according to the present invention.
The object of the present invention is to provide such very thin printer assembly and printer, and it is preferable that the diameter of the platen roller is made as small as possible. Accordingly, it is difficult to provide rigidity to the platen roller so it is not flexed or to maintain the platen roller horizontally relative to the thermal head, as mentioned above. However, in the present invention, the platen roller becomes brought into close contact with the thermal head by the flexibility of the platen roller itself. Namely, since the platen roller and the supporting rollers are flexible, the platen roller can be brought into close contact flexibly with the thermal head even if the thermal head becomes flexible, which is different from a conventional structure. When the entire structure of the printer assembly is made thin, it becomes difficult to reinforce the line thermal head as rigid enough. Therefore, when a thin thermal head is employed, the thermal head itself may be flexed due to pressure. Even in this case, in the printer assembly of the present invention, the platen roller can be brought into close contact with the thermal head. Therefore, this invention provides a thin and reliable printer assembly capable of performing a high quality printing. In addition, as mentioned above, for feeding the sheet or paper reliably in the thin space, combination of the platen roller having a flexible shaft coated or covered with rubber, and the supporting rollers having flexible shafts with low coefficient of friction surface is suitable.
In the printer assembly of the present invention, the platen roller is supported substantially continuously along longitudinal direction via the supporting rollers that extend along the platen roller. Therefore, the platen roller touches continuously, without deformation, to the line thermal head so as to make uniformity print. If the supporting rollers are not continuously extended along the platen roller, surface of the platen roller deforms due to discontinuous pressure from the supporting roller.
In the present printer assembly, pressure for pressing the platen roller to the head can be applied from an upward and/or a downward, namely from the direction of the line thermal head and/or the direction of the supporting plate. It is possible to dispose the pressurizing means between the supporting rollers and the supporting plate. In this case, it is preferable to use thin pressurizing means such as arch-type leaf spring(s). When the supporting rollers touch with the pressurizing means, the supporting rollers rotate can be rotated by low power due to their low friction coefficient.
It is also possible to apply pressure on the platen roller via the supporting plate by the pressurizing means such as a member or members of high elasticity such as springs or rubbers. The supporting plate can be the high rigid plate or, alternatively, the supporting plate can be made of a resilient material such as a leaf spring and serves as the pressurizing means.
When pressure is applied to the platen roller from the side of the supporting plate (from the side opposed to the thermal head), the platen roller contacts closer to the line thermal head by segmenting or dividing the supporting plate along its longitudinal direction at a proper pitch. Particularly, the contacting of the platen roller and the thermal head is increased by combining the segmented supporting plate, the flexible platen roller and the flexible supporting rollers, and it becomes possible to provide the printer assembly and the printer for performing more excellent quality printing.
Undulation comprising alternate projections and recesses or lands and grooves may be formed on the supporting plate along the longitudinal direction of the supporting rollers for reducing frictional force between the supporting rollers and the supporting plate.